Fluidized catalytic cracking (FCC) processes are widely used for the conversion of hydrocarbon feed streams such as vacuum gas oils and other relatively heavy oils into lighter and more valuable hydrocarbon products. The FCC process utilizes a finely divided particulate catalyst fluidized by a gas or vapor for contact with the starting hydrocarbon feed stream, also in a fluidized form. As the particulate catalyst proceeds in the reaction its catalytic sites are covered by coke, a by-product of the reaction, deposited on the surface of the catalyst particles which inhibits the catalytic activity. A catalyst regenerator is used to burn the coke off of the catalyst for regeneration and reuse of the catalyst in the cracking process.
Burning of the coke from the spent catalyst generates large amounts of heat which is utilized at least in part to supply the heat necessary for the endothermic cracking reaction taking place in the reactor. As the hydrocarbon feeds become heavier, i.e. have higher Conradson Carbon values, however, the amount of coke by-product developed on the catalyst in the catalytic reaction increases. Therefore, the use of heavier feeds can lead to excess heat generated during catalyst regeneration due to the burning of larger amounts of coke developed on the catalyst.
The additional heat can create a number of problems in the FCC process, including upsetting the heat balance, requiring limitation of hot catalyst fed to the reaction resulting in lower yields, and damaging the equipment or catalyst. Therefore, it is beneficial to have a means to lower the catalyst temperature during regeneration if the heat balance is exceeded.
Various methods of removing heat during regeneration have been tried, however, heat exchange through indirect contact with a cooling medium has been most widely adopted. Generally, indirect contact heat exchange is achieved using cooling coils or tubes, through which a cooling fluid is passed. The cooling coils can run through a bed of the catalyst particles internal to the regenerator or through a separate catalyst bed external to the regenerator.
Heat exchangers utilizing cooling coils or tubes running through a fluidized catalyst particle bed internal to the regenerator are illustratively shown in U.S. Pat. Nos. 4,009,121 to Luckenbach, 4,220,622 to Kelley, 4,388,218 to Rowe and 4,343,634 to Davis. Internal heat exchangers, however, are difficult to retrofit and/or service.
External heat exchangers are generally flow-through coolers where catalyst is withdrawn from the regenerator and directed into a separate vessel having cooling tubes or coils therein. There are basically two types of external coolers, flow-through and back-mix coolers. Generally, flow-through coolers are either gravity feed, where catalyst enters one upper inlet and exits a lower outlet, or fluidized transport which moves catalyst from a lower inlet past the cooling coils to an upper outlet. Back-mix coolers utilize a common catalyst inlet and outlet to move the catalyst from the hot catalyst source to the heat exchanger and back.
Back-mix heat exchangers are shown in U.S. Pat. Nos. 3,672,069 to Reh et al and 4,439,533 and 4,483,276 both to Lomas et al. U.S. Pat. No. 5,027,893 to Cetinkaya et al relates to a heat exchanger with a combination of back-mix and flow-through characteristics, the inlet being at the top of the exchanger, the outlet in the middle with cooling coils and catalyst throughout the exchanger.
Also, several references disclose a hot catalyst inlet at the mid-portion of the heat exchanger and an outlet at the bottom of the heat exchanger where a fluidizing gas moves the cooled catalyst back up to the regenerator vessel. These illustratively include U.S. Pat. Nos. 2,735,802 to Jahnig and 4,615,992 to Murphy.
Other flow-through heat exchangers, are placed between the hot catalyst source (regenerator) and the reaction zone to regulate the temperature of the catalyst entering the reaction. Examples of such a system are found in U.S. Pat. Nos. 4,284,494 and 4,325,817 to Bartholic et al.
A regenerator apparatus using pure gravity feed flow-through heat exchanger is shown in U.S. Pat. No. 2,970,117 to Harper. The Harper heat exchanger removes catalyst from the catalyst bed of a single stage regeneration vessel and returns the cooled catalyst at a lower portion of the catalyst bed.
A regenerator apparatus using fluidized transport to move catalyst from the bottom of a single stage regenerator upward over the cooling coils and back to the top of the regenerator is described in U.S. Pat. No. 4,064,039 to Penick.
A two-stage regeneration system with catalyst cooling is described in U.S. Pat. No. 4,965,232 to Mauleon et al where regenerated catalyst is removed from the second stage and sent to a holding vessel where it is then sent to an external heat exchanger and cooled catalyst is returned to the first stage of the regeneration zone.
Regulation of the amount of cooling in the heat exchangers is achieved in various ways. For instance, U.S. Pat. Nos. 4,434,245, 4,353,812 and 4,439,533 disclose hydrocarbon conversion processes wherein the catalyst is removed from a regenerator and cooled in side or external heat exchange coolers and then returned to the regenerator. The method described for controlling heat removal in the regenerator involves the extent of immersion of the cooling coils in the dense phase regenerated catalyst bed or controlling the rate of flow of regenerated catalyst through the external coolers.
U.S. Pat. No. 2,436,927 discloses a fluidized catalytic conversion process wherein the crude feed is contacted with a silica-alumina type catalyst for producing high quality gasoline. Heat removal is achieved through the use of an external cooler and control is achieved by regulating the amount of catalyst passing through that cooler.
U.S. Pat. Nos. 3,990,992 and 4,219,442 illustrate regenerator units having heat removal means different from those described above. These regenerator units are divided into two portions, the regenerator having a lower portion for effecting combustion of the catalyst and an upper section wherein residual combustion is effected along with heat removal. Heat removal is achieved through internal coils in the upper section of the regenerator. Temperature control is achieved by controlling the amount of regenerated catalyst removed to the upper zone and then reintroduced along with coke contaminated catalyst to the combustion zone. The balance of the regenerated catalyst is reintroduced to the catalytic reactor.